1. Field of the Invention
The present invention relates to rail or freight car brake systems and, more particularly, to a brake cylinder limiting valves for an AAR-type freight car brake that prevents over-pressurization of the brake cylinder.
2. Description of the Related Art
Control valves used in freight car brake systems, such as the DB-60 control valve manufactured by New York Air Brake Corporation of Watertown, N.Y., or the AB-type control valves manufactured by Wabtec Corporation of Wilmerding, Pa., typically supply air pressure to the brake cylinder of a freight car. If the brake cylinder or the plumbing between the control valve and the car has a leak, however, the brake cylinder will not maintain the original set pressure. In addition to brake cylinder leakage, resulting in low brake cylinder pressure, the brake system can leak into the brake cylinder, resulting in high brake cylinder pressure.
Brake control systems on rail or freight cars that comply with AAR standards are referred to as displacement type system and the brake cylinder pressure is proportional to the size of the auxiliary reservoir and brake cylinder volumes, which are proscribed by AAR regulations and controlled by means of the brake control valve. Control of the brake cylinder pressure is in response to modulation of the brake pipe pressure by the train driver. Although these systems are very reliable, they operate in an open loop mode with the brake cylinder pressure being the result of the relationship between auxiliary reservoir and brake pipe pressures. As a result, there is no feedback of brake cylinder pressure for the purpose of closed loop control. Leakage into or out of the brake cylinder may therefore result in brake cylinder pressures that are higher or lower than desired without any recognition by the system that the pressures are abnormal. While recently improvement to AAR brake systems include the addition of brake cylinder maintaining valves that compensate for brake cylinder leakage, the issue of brake cylinder over-pressurization is still a problem and may occur as the result of leakage in the quick service limiting valve, in the auxiliary reservoir, in the emergency reservoir, in the auxiliary reservoir, or in the brake pipe pressure into the brake cylinder while the brakes are applied.
A brake failure that results in over-pressurization of the brakes on a car in train is very hazardous and may result in “hot wheels,” which damages the wheels and raises the potential for a subsequent wheel failure and even train derailment. The train driver is usually unaware that a car has over-pressurized brakes due, in part, to the length of the train and the number of cars in the train. The only existing method of addressing this problem is to install a network of hot wheel detectors along a predetermined location in the continental rail system that can detect a hot wheel on a car using a thermal sensor, identify the car ID using an RFID tag, and then send an alarm to a dispatch center so that a dispatcher can contact the train driver. Such systems are costly, require significant modifications to the existing infrastructure, and are limited in geographic scope. As a result, rail car mounted system that can prevent over-pressurization of the brake cylinder and avoid the resulting hot wheel problem would be a significant safety improvement.